Adding DC voltage signals

Building a frequency offset locking circuit for laser spectroscopy. First electronics project so been hitting a steep learning curve. Long story short, I have two branches in this circuit that take a AC signal run it through a diode and low pass filter to measure the envelope and output a DC voltage. The diode in the second branch is inverted so it gives me a negative voltage. At the end of the two branches I need a way to connect them so that the output is a sum of the 2 DC voltages...eg, when both branches are .2mV (one +, one -) the output is 0, as its being used as the error signal to the laser servo. The paper I'm working off uses a resister after each filter and then one more resister after the branches are connected...would that work? What principle is it using so I can figure out how to choose the values of the resister based on modifications I've made to the circuit design.

Unless you need to reduce the output voltage, the "...one more resistor after the branches are connected" generally isn't needed.

Just use the same resistor values on each of the filter outputs... or if this is not a production item and the filters don't quite match, change a resistor value as needed for a better match.
A summing amplifier is just your two resistors with gain added if needed. If you need a higher voltage for the next stage or if the next stage loads down the signal too much, make it a summing amplifier.